Those tree-chom­pers may not be canker­worms

Winnipeg Free Press - Section G - - HOMES - MICHAEL ALLEN

USERS of Tan­gle­foot tree bands are un­der­stand­ably per­plexed when they still get heavy feed­ing by canker­worms.

One ex­pla­na­tion may be that they don’t have canker­worms at all. Elm span­worms cause sim­i­lar feed­ing dam­age to canker­worms and are not pre­vented from get­ting into trees by Tan­gle­foot bands.

The span­worm, a type of cater­pil­lar, is larger than the canker­worm and usu­ally varies from a mot­tled, yel­low­ish grey-brown to black in colour, al­though I have only seen the darker-coloured worms in the Win­nipeg area in re­cent years. The head and rear parts are a dis­tinct red­dish brown in colour. Canker­worms can be green or black in colour.

Span­worms are of­ten two or three times larger than canker­worms. The “worm” is ac­tu­ally called a larva, and it is only one stage in the devel­op­ment of this in­sect. Af­ter about four weeks of feed­ing on leaves it will un­dergo a phys­i­cal change and be­come a pupa rest­ing in a net-like co­coon that it spins. Later in sum­mer the pupa changes into a snowy white moth.

I have seen these span­worm moths gather in large num­bers on the bark of oak trees. The fe­male moths usu­ally lay their eggs on the un­der­side of twigs and in deep crevices of tree bark. The eggs hatch the fol­low­ing spring to be­come the fa­mil­iar span­worm lar­vae.

Al­though they are called elm span­worms, they do feed on other tree species, es­pe­cially oak, maple and ash. They also are some­what com­mon on fruit trees.

Un­like canker­worm moths, both the fe­male and male span­worm moths fly, so they would not be trapped by sticky Tan­gle­foot bands. On oc­ca­sion, it’s pos­si­ble to see both canker­worms and elm span­worms feed­ing on the same tree.

Elm span­worms are best con­trolled while they are young by a bi­o­log­i­cal pes­ti­cide spray such as BtK (eg. Dipel).

There’s an­other pos­si­ble rea­son why canker­worms are feed­ing on trees that have been banded. If your neigh­bours’ trees or nearby boule­vard trees have not been Tan­gle­foot-banded, the canker­worms can crawl from those tree branches that touch your tree branches. More com­monly, canker- worms at­tached to the end of their long threads can swing into your trees when it’s windy and sim­ply start feed­ing once they land.


Elm span­worm, right, feeds vo­ra­ciously on leaves and can be con­fused with canker­worms. Left: an adult span­worm moth.

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